Horn antenna having plural convergent waveguide paths



April 8, 1969 M. F. RADFORD 3,438,040

HORN ANTENNA HAVING PLURAL CONVERGENT WAVEGUIDE PATHS Filed Sept. 26, 1965 Sheet 1 01' 2 FIG/ lNvENToR A TQRNEY April 1969 M. F. RADFORD 3,438,040

HORNANTENNA HAVING PLURAL CONVERGENT WAVEGUIDE PATHS Filed Sept. 26. 1956 Sheet 2 Of 2 LL u lNvEN'roP- meg/ 1; 2W KW mm, WW, may 6 54020721 ATTORNEY United States Patent Office 3,438,040 Patented Apr. 8, 1969 3,438,040 HORN ANTENNA HAVING PLURAL CON- VERGENT WAVEGUIDE PATHS Mathew Frederick Radford, Danbury, Essex, England, as-

signor to The Marconi Company Limited, London, England, a British company Filed Sept. 26, 1966, Ser. No. 581,954 Claims priority, application Great Britain, Oct. 15, 1965, 43,919/ 65 Int. Cl. H01q 13/00, 13/02, 13/04 U.S. Cl. 343778 12 Claims This invention relates to directional aerials and has for its object to provide improved directional aerials which are suitable for use in radar systems and shall have a direction of maximum directivity which is substantially independent of frequency over its useful range and which shall also have wide band characteristics.

It is common at the present timeto employ linear arrays of the slotted wave guide type as the aerials of radar systems. Such arrays have the defect that the direction of maximum directivity is seriously dependent on frequency. This defect is often very objectionable. For example it makes such aerials unsuitable for use in frequency diversity radar systems and in such systems, accordingly substantially point sources of radiation are commonly used in conjunction with reflectors of double curvature despite that such reflectors are expensive and difiicult to design and construct. More complex aerial systems without this defect are known-for example arrays of separate radiators fed through power dividing feeder networksbut have the disadvantages of high cost and complexity. Moreover they are, in general, narrow band. This invention seeks to provide improved and simple wide band aerials without the foregoing defect.

According to this invention a directional aerial comprises means providing a plurality of pairs of convergent wave guide paths opening into a common aerial aperture surface, the ends of said paths remote from said surface opening into a further wave guide path having a central feed, the paths of each said convergent pair being at opposite acute angles to said further path and the overall path lengths from said central feed to the ends of said convergent paths in said aperture surface being substantially the same throughout.

The common aerial aperture may be a planar rectangular aperture but preferably it is composed of two planar rectangular portions which meet at a common shorter side and at obtuse angle at the middle of the aperture so that said aperture is swept back from its centre to its ends.

Preferably the convergent wave guide paths are provided by means of inclined plates extending from the aperture surface to the further wave guide path and which are substantially parallel at least over a portion of their lengths starting from the further wave guide. These plates may be parallel over their whole lengths but, preferably, their ends near the aperture are curved to provide curved matching sections.

A preferred construction is of very approximately segmental shape defined by two side plates, and an angled back plate having a straight, in-line, central portion which is symmetrical about the central feed and continues into two similar straight outer portions extending to the aerial aperture and at opposite obtuse angles with said central portion, there being provided between said side plates, a central structure which at least approximates in shape to that of an isosceles triangle and has its base spaced from parallel to and substantially co-extensive with the in-line central portion of said back plate, there being also provided between said side plates a plurality of spaced plates having at least a portion of their lengths parallel to one side of said triangular structure and extending from the aperture to an imaginary surface spaced from one of said outer portions of the back plate and a symmetrically arranged like plurality of spaced plates having at least a portion of their lengths parallel to the other side of said structure and similarly extending from the aperture to an imaginary surface similarly spaced from the other of said outer portions of the back plate, the lengths of the acute angled paths thus provided from the feed to the aperture surface being all of substantially the same length.

Preferably the ends of the plates near the aperture are curved so as to provide curved matching sections; the side plates are so shaped that the aperture surface is composed of two rectangular surface portions which meet at a common shorter side at an obtuse angle so that said aperture is swept back from the centre to its ends by amount of between a quarter of a wave length and one wavelength; and the sides of the approximately triangular structure are curved near the apex so as to meet at a very acute angle at the centre of the aperture.

The side plates may be parallel to one another or they may be divergent in the direction of the aerial aperture so that the approximately segmental aerial is flared.

The feed may be a simple central feed provided by a wave guide opening into the middle of the length of the further wave guide path, i.e. in the preferred cheese-like embodiments, opening into the middle of the length of the space between the in-line central portion of the back plate and the base of the at least approximately triangular structure. Alternatively there may be a separate feed to each half of the aerial, the middle of the length of the further wave guide path being interrupted and the two inner ends of the two paths produced by said interruption connected to two feeders fed through a 3 db coupler.

From the point of view of good impedance matching the best arrangement is that in which the spacing, measured in the aerial aperture surface, between the ends of adjacent paths terminating in that surface is half a wave length and the spacing between the other ends of said paths terminating in the aforesaid further wave guide path is a quarter of a wave length. However acceptably good impedance matching is obtainable together with a substantial and worthwhile reduction in size if the former spacing is increased to approximately two-thirds of a wave length and the latter spacing to one half of this. The worst arrangement, from the point of view of impedance matching, is that in which the former spacing is increased to one wave length and the latter to half this and such dimensioning should be avoided in carrying out the invention.

The invention is illustrated in and further explained in connection with the accompanying drawings. In the drawings:

FIGURES 1 and 2 are mutually perpendicular views of one embodiment;

FIGURE 3 is a part view of the same nature as FIG- URE 2, illustrating a modification;

FIGURE 4 is a View showing a preferred embodiment; and

FIGURE 5 is a diagram showing two of the paths through the aerial of FIGURE 4.

Referring to FIGURES 1 and 2, the aerial therein shown is of very approximately segmental form. The sides are defined by two side plates 1 and 2 which, in the example illustrated, are parallel to one another though they could be divergent in the direction of the aerial aperture if a flared construction is required. The back of the aerial is defined by an angled back plate consisting of a straight, in-line central portion 3 which continues into two similar straight outer portions 4 and 5 extending to the aperture of the aerial which is planar and rectangular.

Between the side plates is an isosceles triangular structure having its base 6 spaced from, parallel to and substantially co-extensive with, the in-line central portion 3 of the back plate. Also between the side plates are a plurality of parallel spaced plates 7 and 8, the plates 7 being parallel to one side 9 of the triangular structure and the plates 8 being similarly parallel to the remaining other side 10 of said structure. Each plate 7 or 8 extends from the aerial aperture to one or other of two imaginary surfaces, one spaced from and slightly inclined with respect to the outer portion 4 of the back plate and the other spaced from and slightly inclined with respect to the outer portion 5 thereof. A single feeder 11 opens into the space between the central portion 3 of the base plate and the base 6 of triangular structure.

It will be seen that with this illustrated arrangement the aerial aperture is rectangular and planar and a plurality of paths exist between the feeder 11 and the said aperture. In the left hand half of the aerial these paths have a common portion between the portion 3 of the back plate and the base 6 of the triangular structure and then turn through acute angles near the outer portion 4 to go through the spaces at either side of each of the plates 7. Similarly in the right hand half of the aerial the paths have a common portion between the portion 3 and the base 6 and then turn through acute angles near the outer portion 5 to go through the spaces on either side of each of the plates 8. All these paths are made of the same length. Accordingly the direction of maximum directivity, which will be quite sharp, is at right angles to the plane of the aerial aperture, the direction of the maximum lobe being perpendicular to said aperture at the middle of its length.

In FIGURE 1 there is a simple central feed. It is obviously possible, however, to use a double feed, i.e. an input arrangement for so-called static split. FIGURE 3 illustrates this. The middle of the common portion of the paths to the aerial aperture is interrupted so as to form two paths, one directed to the left and the other to the right, with their adjacent ends close together. These paths are fed by two waveguides, one of which is referenced 11' and the other 11". These waveguides are fed via a 3 db coupler 12, the right hand input being applied via the wave guide 13 and the left hand input being applied via the wave guide 14.

Aerials in accordance with this invention may be constructed to operate at any of a wide range of frequencies and are particularly suitable for use at frequencies in the range of 1,000 mc./s. to 10,000 mc./s. They are remarkably free from frequency-dependent directivity. Moreover, as compared with the commonly employed linear arrays of the slotted wave guide type, they are wide band. Experiment indicates that a bandwidth ratio of 3:2 is readily obtainable. This compares very favourably with the bandwidth ratio of about 11:10 which is approximately all that is generally obtained with linear arrays of the slotted wave guide type. Also it will be noted that the structure is simple and strong mechanically. If the aerial is to be used in conjunction with a reflector considerable economy in the reflector can be achieved since it does not have to be of double curvature in order to deal with frequency-dependent directivity. Obviously a number of aerials in accordance with the invention may be used conjointly, e.g. end to end with their apertures in a common plane, in order to obtain high powers.

The embodiments of FIGURES 1 to 3 may with advantage be modified in such manner that the paths (of equal length) through the aerial are continued by extending the plates 7 and 8 into extensions each approximately of a wavelength long, lying in planes at right angles to the plane of the aperture and positioned between the side plates 1 and 2. In this construction, therefore, each of the equal length paths is bent through an angle near the aerial aperture. An additional extension plate parallel to the other extensions extends from the apex of the triangular structure. In order to facilitate good impedance matching between the ends of said paths and the aperture, matching means are preferably provided at or near the angles in the separate paths. Thus, the change in direction between the plates 7 and 8 and the extension plates may be made gradually e.g. by curving the plates where the angular change occurs or altering the plate directions in that position in a series of small steps.

The mere addition of curved impedance matching sections at the aperture ends of the plates 7 and 8 has been found insuflicient to give a really good impedance match over the whole working band of the aerial because the reflections from the aerial, though small, add up in phase at the feeder terminal. A substantial improvement can be obtained, however, by constituting the aperture by two similar rectangular portions which meet at an obtuse angle at a common shorter side so that the aperture is swept back from the centre to the outer to cause the latter to be behind the centre. The amount of sweep back may be in practice from about a quarter of a wavelength to a wavelength. FIGURE 4 shows a preferred embodiment of this nature. As will be seen the plates 7 and 8 are curved at their aperture ends to form curved matching sections and the aerial aperture is swept back by an amount indicated by the dimension S. A small change (compared to FIG- URES 1 and 2) is made in the geometry of the platesthere being a small departure from the isosceles triangle shape of FIGURES l and 2to correct for defocussing error introduced by the sweep back. The apex end of the sides 9, 10 of the triangular structure are curved to come together at a very acute angle. In FIGURE 4 the reflections from the aperture ends of the paths are staggered without appreciably interfering with the frequency independence. FIGURE 4, the right hand half of which is drawn with the side plate (the top one in FIGURE 4) removed to show the plates 8, the parts below the top plate being shown in broken lines in the left hand half of the figure. Also in FIGURE 4, is indicated a flare FL formed by extending the top and bottom side plates (corresponding to the plates 1 and 2 of FIGURE 1) beyond the ends of the plates 7 and 8 and diverging them.

FIGURE 4 is a diagram showing two of the paths (the extreme left and right hand paths through the right hand half of the aerial of FIGURE 4) through the aerial of FIGURE 4. For simplicity of drawing the parts of the paths are represented as though they were all straight. Different points on these paths are indicated by reference letters which are also shown in FIGURE 4. It may be shown that, for perfect compensation Where kg. is the guided wave length and A the wavelength in free space. Theoretically this question can be satisfied only at one frequency (the mid-band frequency) since A is not proportional to Ag. However the error is negligible over a practically adequate wide band, since the amount of sweepback, EF, is relatively small.

In FIGURE 4 there is shown a simple central feed as in FIGURE 2. Obviously a double feed, as in FIGURE 3, providing static split, could be used.

I claim:

1. A directional aerial comprising means providing a plurality of pairs of convergent wave guide paths opening into a common aerial aperture surface, the ends of said paths remote from said surface opening into a further wave guide path having a central feed, the paths of each said convergent pair being at opposite acute angles to said further path and the overall path lengths from said central feed to the ends of said convergent paths in said aperture surface being substantially the same throughout.

2. An aerial as claimed in claim 1 wherein the common aerial aperture is planar and rectangular.

3. An aerial as'claimed in claim 1 wherein the common aerial aperture is composed of two planar rectangular portions which meet at a common shorter side and at an obtuse angle at the middle of the aperture so that said aperture is swept back from its centre to its ends.

4. An aerial as claimed in claim 1 wherein the convergent waveguide paths are provided by means of inclined plates extending from the aperture surface to the further waveguide path and which are substantially parallel at least over a portion of their lengths starting from the further waveguide.

5. An aerial as claimed in claim 1 wherein the convergent waveguide paths are provided by means of inclined plates extending from the aperture surface to the further waveguide path and which are substantially parallel over these portions near the further waveguide and continue into curved portions which are curved to provide curved matching sections.

6. A very approximately segmentally shaped aerial as claimed in claim 1 and having its shape defined by two side plates, and an angled back plate having a straight, in-line, central portion which is symmetrical about the central feed and continues into two similar straight outer portions extending to the aerial aperture and at opposite obtuse angles with said central portion, there being provided between said side plates, a central structure which at least approximates in shape to that of an isosceles triangle and has its base spaced from, parallel to and substantially co-extensive with the in-line central portion of said back plate, there being also provided between said side plates a plurality of spaced plates having at least a portion of their lengths parallel to one side of said triangular structure and extending from the aperture to an imaginary surface spaced from one of said outer portions of the back plate and a symmetrically arranged like purality of spaced plates having at least a portion of their lengths parallel to the other side of said structure and similarly extending from the aperture to an imaginary surface similary spaced from the other of said outer portions of the back plate, the lengths of the acute angled paths thus provided from the feed to the aperture surface being all of substantially the same length.

7. An aerial as claimed in claim 3 wherein the amount of sweep back is between one quarter of a wavelength and one wavelength.

8. An aerial as claimed in claim 6 wherein the ends of the plates near the aperture are curved so as to provide curved matching sections; the side plates are so shaped that the aperture surface is composed of two rectangular surface portions which meet at a common shorter side at an obtuse angle so that said aperture is swept back from the centre to its ends by an amount of between a quarter of a wave length and one wave length; and the sides of the approximately triangular structure are curved near the apex so as to meet at a very acute angle at the centre of the aperture.

9. An aerial as claimed in claim 6 wherein the side plates are parallel to one another.

10. An aerial as claimed in claim 6 wherein at least portions of the side plates diverge to provide the aerial with a flare.

11. An aerial as claimed in claim 1 wherein the feed is a simple central feed provided by a Wave guide opening into the middle of the length of the further wave guide path.

12. An aerial as claimed in claim 1 wherein there is a separate feed to each half of the aerial, the middle of the length of the further wave guide path being interrupted and the two inner ends of the two paths produced by said interruption connected to two feeders fed through a 3 db coupler.

References Cited UNITED STATES PATENTS 2,628,311 2/1953 Lindenblad 343786 FOREIGN PATENTS 894,342 4/ 1962 Great Britain.

ELI LIEBERMAN, Primary Examiner.

US. Cl. X.R. 343783, 786 

1. A DIRECTIONAL AERIAL COMPRISING MEANS PROVIDING A PLURALITY OF PAIRS OF COVERGENT WAVE GUIDE PATHS OPENING INTO A COMMON AERIAL APERTURE SURFACE, THE ENDS OF SAID PATHS REMOTE FROM SAID SURFACE OPENING INTO A FURTHER WAVE GUIDE PATH HAVING A CENTRAL FEED, THE PATHS OF EACH SAID CONVERGENT PAIR BEING AT OPPOSITE ACUTE ANGLES TO SAID FURTHER PATH AND THE OVERALL PATH LENGTHS FROM SAID CENTRAL FEED TO THE ENDS OF SAID CONVERGENT PATHS IN SAID APERTURE SURFACE BEING SUBSTANTIALLY THE SAME THROUGHOUT. 